In the combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as a power plant, a hot process gas is generated, such process gas containing, among other components, carbon dioxide CO2. With increasing environmental demands various processes for removing carbon dioxide from the process gas have been developed. One such process is the so called oxy-fuel process. In an oxy-fuel process a fuel, such as one of the fuels mentioned above, is combusted in the presence of a nitrogen-lean gas. Oxygen gas, which may provided by an air separation unit, is supplied to a boiler in which the oxygen gas oxidizes the fuel. In the oxy-fuel combustion process a carbon dioxide rich flue gas is produced, which can be treated using various CO2 capture technologies in order to reduce the emission of carbon dioxide into the atmosphere.
Further, CO2 capture often comprises cooling, or compression and cooling, of the flue gas to separate CO2 in liquid form from non-condensable flue gas components, such as N2 and O2.
After purification and separation of carbon dioxide, a carbon dioxide rich stream is obtained and need to be handled, such as by storing and transportation in tanks (stationary or on a truck or ship), transporting via pipelines and/or pumping into the ground for prolonged (definitive) storage and mineralisation.
Different components used in an oxy-fuel process may not always be used to their full capacity. Components downstream of the boiler are designed in view of the output from the boiler. Some of the apparatuses used in an oxy-fuel process are thus oversized since the oxy-fuel process not always is operated at full capacity all the time. One such apparatus may be the compressors in the gas processing unit (GPU) acting on the carbon dioxide rich flue gas stream. The operating range of any compressor is limited on the high flow side of the choke region and on the low flow side by surge. For example big centrifugal compressors are limited in turn down to about 75% of their nominal capacity. To achieve a further capacity reduction and avoid surge bypass arrangements may be used, wherein a part of the compressed carbon dioxide stream exiting a compressor is recirculated back to the inlet of the compressor in order to keep a certain volume flow over the compressor. However, in the event of compressor failure such a bypass is normally fully opened to ensure the gas flow to the machine. In oxyfuel systems compressor failure resulting in fully opened bypass would lead to pressure peaks within the system which may damage the equipment and thus affect the entire power plant. Also, the inventory stored downstream of the compressor discharge may be enough to lead to pressure peaks that change the power plant regime from underpressure to overpressure when recirculated back via the fully opened bypass.
The issue with pressure peaks arising upon compressor failure is at present a problem. A pressure peak may also adversely affect the flame stability in the oxyfuel boiler. It is to be noted that compressor trips most likely will lead to power plant trips. Currently there are no systems on the market that have a reliability to prevent potential peaks.
There is always a need to improve the flexibility of an oxy-fuel process. It would be desirable to find new ways to ensure more a stable operation, scale down the size/capacity of the components and better and more securely utilize the components present in an oxy-fuel process.